@willcroft @jensimmons I ended up submitting this as FB20414621 because it seems to be getting worse. Today Safari 26 even crashed after beachballing for a long time while trying to type in the address bar.
The bad performance seems to get worse in relation to how long Safari has been open or how much browsing has been done, so initially everything seems fine, but after a few hours it sometimes takes seconds to respond (on an M4 Pro!).